[ 3 2 9 ] 
• From this difference in the power at different di- 
ftances, I inferred, that the Tourmalin rejijh the exit 
and entrance of the fluid conflderably lefs than glafs y or 
even amber ; for the diftances requifite to caufe 
changes in thefe, were lefs, than the diftances which 
caufed changes in the Tourmalin : and fince, by ex- 
pofing glafs, and the Tourmalin , to an eledrified body , , 
at confiderable diftances, they are rendered minus, 
and at a nearer diftance plus ; it is no inconfiderable 
argument that their general laws are the fame ; and 
that the Tourmalin differs in ?io thing fro?n other elec- 
tric bodies , but in acquiring an eleBricity by heat. 
And, in regard to this remarkable effed, the experi- 
ments I formerly made, which rendered eledrics 
non-eledric, as likewife Mr. Delaval’ s curious expe- 
riments upon earthy fubftances, are other inftances 
how particular bodies may be fo altered, as to fuffer 
the eledric fluid to pafs through them or not, accord- 
ing to the different degrees of heat employed in the 
experiment. 
But to proceed with our obfer vations on rubbed gl afs. 
Exp. 27. Having by me a pane of glafs, one fide 
of which was rough, and the other fmooth, I rubbed 
it (lightly on the rough fide ; upon doing which, both 
fides were eleftrified minus. 
Exp. 28. I treated the other fide in the fame man- 
ner; after which, the minus eledricity was changed 
to a plus one , on both fldes. 
Now, becaufe the fame glafs afforded different ap- 
pearances when different fides thereof were rubbed, 
and there being no other difference in the circum- 
ftances of the two experiments, except that in the 
furfaces themfelves, the one being rough, and the 
Vol, LI. L T u • other 
